man3/adjtime.3

   1 '\" t
   2 .\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH adjtime 3 (date) "Linux man-pages (unreleased)"
   7 .SH NAME
   8 adjtime \- correct the time to synchronize the system clock
   9 .SH LIBRARY
  10 Standard C library
  11 .RI ( libc ", " \-lc )
  12 .SH SYNOPSIS
  13 .nf
  14 .B #include <sys/time.h>
  15 .PP
  16 .BI "int adjtime(const struct timeval *" delta ", struct timeval *" olddelta );
  17 .fi
  18 .PP
  19 .RS -4
  20 Feature Test Macro Requirements for glibc (see
  21 .BR feature_test_macros (7)):
  22 .RE
  23 .PP
  24 .BR adjtime ():
  25 .nf
  26     Since glibc 2.19:
  27         _DEFAULT_SOURCE
  28     glibc 2.19 and earlier:
  29         _BSD_SOURCE
  30 .fi
  31 .SH DESCRIPTION
  32 The
  33 .BR adjtime ()
  34 function gradually adjusts the system clock (as returned by
  35 .BR gettimeofday (2)).
  36 The amount of time by which the clock is to be adjusted is specified
  37 in the structure pointed to by
  38 .IR delta .
  39 This structure has the following form:
  40 .PP
  41 .in +4n
  42 .EX
  43 struct timeval {
  44     time_t      tv_sec;     /* seconds */
  45     suseconds_t tv_usec;    /* microseconds */
  46 };
  47 .EE
  48 .in
  49 .PP
  50 If the adjustment in
  51 .I delta
  52 is positive, then the system clock is speeded up by some
  53 small percentage (i.e., by adding a small
  54 amount of time to the clock value in each second) until the adjustment
  55 has been completed.
  56 If the adjustment in
  57 .I delta
  58 is negative, then the clock is slowed down in a similar fashion.
  59 .PP
  60 If a clock adjustment from an earlier
  61 .BR adjtime ()
  62 call is already in progress
  63 at the time of a later
  64 .BR adjtime ()
  65 call, and
  66 .I delta
  67 is not NULL for the later call, then the earlier adjustment is stopped,
  68 but any already completed part of that adjustment is not undone.
  69 .PP
  70 If
  71 .I olddelta
  72 is not NULL, then the buffer that it points to is used to return
  73 the amount of time remaining from any previous adjustment that
  74 has not yet been completed.
  75 .SH RETURN VALUE
  76 On success,
  77 .BR adjtime ()
  78 returns 0.
  79 On failure, \-1 is returned, and
  80 .I errno
  81 is set to indicate the error.
  82 .SH ERRORS
  83 .TP
  84 .B EINVAL
  85 The adjustment in
  86 .I delta
  87 is outside the permitted range.
  88 .TP
  89 .B EPERM
  90 The caller does not have sufficient privilege to adjust the time.
  91 Under Linux, the
  92 .B CAP_SYS_TIME
  93 capability is required.
  94 .SH ATTRIBUTES
  95 For an explanation of the terms used in this section, see
  96 .BR attributes (7).
  97 .ad l
  98 .nh
  99 .TS
 100 allbox;
 101 lbx lb lb
 102 l l l.
 103 Interface       Attribute       Value
 104 T{
 105 .BR adjtime ()
 106 T}      Thread safety   MT-Safe
 107 .TE
 108 .hy
 109 .ad
 110 .sp 1
 111 .SH STANDARDS
 112 None.
 113 .SH HISTORY
 114 4.3BSD, System V.
 115 .SH NOTES
 116 The adjustment that
 117 .BR adjtime ()
 118 makes to the clock is carried out in such a manner that the clock
 119 is always monotonically increasing.
 120 Using
 121 .BR adjtime ()
 122 to adjust the time prevents the problems that can be caused for certain
 123 applications (e.g.,
 124 .BR make (1))
 125 by abrupt positive or negative jumps in the system time.
 126 .PP
 127 .BR adjtime ()
 128 is intended to be used to make small adjustments to the system time.
 129 Most systems impose a limit on the adjustment that can be specified in
 130 .IR delta .
 131 In the glibc implementation,
 132 .I delta
 133 must be less than or equal to (INT_MAX / 1000000 \- 2)
 134 and greater than or equal to (INT_MIN / 1000000 + 2)
 135 (respectively 2145 and \-2145 seconds on i386).
 136 .SH BUGS
 137 A longstanding bug
 138 .\" http://sourceware.org/bugzilla/show_bug?id=2449
 139 .\" http://bugzilla.kernel.org/show_bug.cgi?id=6761
 140 meant that if
 141 .I delta
 142 was specified as NULL,
 143 no valid information about the outstanding clock adjustment was returned in
 144 .IR olddelta .
 145 (In this circumstance,
 146 .BR adjtime ()
 147 should return the outstanding clock adjustment, without changing it.)
 148 This bug is fixed
 149 .\" Thanks to the new adjtimex() ADJ_OFFSET_SS_READ flag
 150 on systems with glibc 2.8 or later and
 151 Linux kernel 2.6.26 or later.
 152 .SH SEE ALSO
 153 .BR adjtimex (2),
 154 .BR gettimeofday (2),
 155 .BR time (7)